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The single order of the mammalian subclass Prototheria. Two living families, the Tachyglossidae and the Ornithorhynchidae, make up this unusual order of quasimammals, or mammallike reptiles.
The Tachyglossidae comprise the echidnas (spiny anteaters), which have relatively large brains with convoluted cerebral hemispheres. The known genera, Tachyglossus and Zaglossus, are terrestrial, feeding on termites, ants, and other insects. They are capable diggers, both to obtain food and to escape enemies. Like hedgehogs, they can erect their spines and withdraw their limbs when predators threaten. Commonly one egg, but occasionally two or even three, is laid directly into the marsupium (pouch) of the mother where it is incubated for up to 10 days. Species of Tachyglossus live in rocky areas, semideserts, open forests, and scrublands. They are found in Australia, Tasmania, New Guinea, and Salawati Island. Species of Zaglossus are found in mountainous, forested areas.
The duck-billed platypus, constituting the Ornithorhynchidae, has a relatively small brain with smooth cerebral hemispheres. The young have calcified teeth, but in the adult these are replaced by horny plates which form around the teeth in the gums. The snout is duck-billed. The semiaquatic platypus is a capable swimmer, diver, and digger. Two eggs are usually laid by the female into a nest of damp vegetation. After incubating the eggs for about 10 days the female leaves, returning only when the eggs are hatched. The platypus is found in Australia and Tasmania in almost all aquatic habitats. See Mammalia, Prototheria
an order of mammals, comprising two families (the Echidnidae, or anteaters, and the platypus), represented by five species.
The Monotremata are the only group of egg-laying mammals. The eggs are rich in nutritive yolk and are encased in a leathery shell. Monotremes have a number of features in common with the reptiles, such as the structure of the shoulder girdle, certain bones of the skull, and the circulatory system and the organization of the eyes and olfactory organs. The intestines and the urogenital sinus do not open independently to the skin surface but, as in reptiles and birds, discharge into a cloaca (hence the Russian kloachnye). The mammary glands have a primitive tubular structure, opening through numerous orifices to special twin glandular fields; there are no nipples. Monotremes are warm-blooded, regulating body temperature in a manner similar to that of other mammals, but their homoiothermy is maintained only as long as the temperature of the environment remains above 26°C. The structure of the fur coat is typical for mammals.
Adult monotremes are toothless. Young platypuses have ten milk teeth, which are similar in structure to the teeth of mammals of the Mesozoic. Monotremes are found in Australia, Tasmania, and New Guinea.